(1) Field of the Invention
The present invention relates generally to heartstimulation devices, and particularly to atrial synchronous heart stimulation devices.
(2) Description of the Prior Art
In the field of medical electronics, heart pacers are becoming widely accepted and well known at this juncture. There are several varieties of heart pacer available on the market today. There are pacers which provide stimulation to the ventricle of the heart at a fixed rate, pacers which provide stimulation to the ventricle of the heart on demand, pacers which provide stimulation to the atrium and ventricle of the heart in a predetermined timing sequence and on demand, and pacers which provide stimulation to the ventricle in response to signal inputs received from the patient's body other than from the ventricular chambers of the patient's heart. (By no means is this listing intended to include all types of pacers available today.) It is with this last mentioned type of heart pacer that the present invention is more particularly related.
In a normally functioning heart, the sinoatrial mode initiates depolarization of the elctrically polarized muscalature of the heart and serves as the normal pacemaker of the heart. The depolarization initially causes contraction of the atria, as represented by the electrical P-wave, and subsequently causes contraction of the ventricles (represented by the QRS complex) following a delay known as the A-V delay. The A-V mode is responsible for the A-V delay which ensures proper asynchronous contraction of the atria and ventricles for efficient cardiac hemodynamics. In fact, the A-V delay comprises by far the major portion of the P-R delay which also includes the time required to excite the atria and the time occupied by conduction from the A-V mode to the ventricular myocardium. However, the A-V delay is not constant at all heart rates. Instead, it varies from a relatively long interval (i.e. 170-250 milliseconds) for slow heart rates (i.e. 55-70 beats per minute) to a relatively short interval (i.e. 125-170 milliseconds) for rapid heart rates (i.e. 130-150 beats per minute), with the A-V delay in the adult human heart tending to be greater than that in a child's heart at any particular heart rate. The pacer of the invention is particularly suited for use when so-called A-V block prevents normal conduction of the depolarizing voltage from the atria to the ventricles.
Detected P-waves representing atrial contractions of the heart have been used to control the timing of ventricular stimulation pulses. Such pacers may be generally designated as being atrial synchronous. The U.S. Pat. No. 3,648,707 which issued on Mar. 14, 1972 is cited as a somewhat limited example of this prior art. That patent describes a device which can detect P-waves and which causes a ventricular pulse generator to supply a stimulation pulse at a fixed interval (A-V interval) of time after the P-wave detection occurred. If no P-wave occurs, the pacer provides a stimulation to the ventricle of the heart at the end of a predetermined interval from the previous ventricular heart beat or stimulation. If a P-wave is sensed at or near the time of occurrence of the ventricular stimulation impulse, the ventricular stimulation impulse still occurs at the expected time, This results in an upper limit on the interval between the ventricular stimulation impulses, or in other words, a lower limit to rate.
More recently, there has been developed a heart pacer which responds to sensed P-waves and includes timing and control means for generating a fixed-interval A-V interval pulse which (unless inhibited by a natural ventricular contraction) acts, at its conclusion, to generate a ventricular stimulation impulse even if the P-wave occurs only shortly before the expiration of the predetermined basic timing interval initiated by the previous ventricular beat or stimulation. This latter pacer is designed such that the basic timing interval provides a practical intermediate or median stimulation rate, which stimulation rate may decrease for relaxation or sleep, and alternatively increase for exercise, as a function of the rate of atrial contractions.
Even though the pacer described immediated above represents a significant advance in heart pacers by its ability to vary the ventricular stimulation rate as a function of the atrial beating rate, it may not exactly stimulate the heart's conduction system because it relies on an A-V interval of fixed duration independent of heart rate. The use of a constant A-V interval may also tend to limit the practical range of heart rates over which the atrial synchronous pacer is operable.
It is, therefore, a principal object of the invention to provide an improved pacer responsive to a sensed P-wave for generating a ventricular stimulation pulse following an A-V delay interval of automatically determined length or duration. Included within this object is the provision of means for automatically determining the interval of the A-V delay as a function of the atrial beating rate.
It is another object of the invention to provide an improved heart pacer which is responsive to atrial contractions and accurately stimulates or approximates the natural conduction system of the heart.